pH-insensitive glucose indicator protein

ABSTRACT

The present invention encompasses a glucose indicator protein, a biosensor comprising one or more glucose indicator proteins, and methods of use thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. provisional application No. 61/250,118, filed Oct. 9, 2009, which is hereby incorporated by reference in its entirety.

GOVERNMENTAL RIGHTS

This invention was made with governmental support under contract number 1R15 EB006378 awarded by the National Institutes of Health. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention encompasses a glucose indicator protein.

BACKGROUND OF THE INVENTION

Diabetes mellitus is one of the major health care problems in the world. It affects 16 million people in the United States and over 100 million people worldwide. More frequent monitoring of blood glucose can prevent many long-term complications associated with diabetes. Although finger-stick testing is currently available for blood glucose monitoring, the nature of this test restricts its utility for maintaining a strict level of blood glucose. This has resulted in a worldwide effort to develop noninvasive methods for fast, painless, and convenient monitoring of glucose.

Most biosensors use proteins, which provide the desired analytic specificity, but often are not appropriate for noninvasive detection because they lack an intrinsic signal transduction mechanism. A variety of glucose binding proteins have been isolated and well characterized. These proteins are highly specific for glucose binding, but do not provide any optical signal in the visible region upon glucose binding.

To date, glucose biosensors are pH sensitive, and will not work properly in certain in vivo applications, where the pH can vary with the metabolic activity of the cell. In addition, the glucose binding proteins used to develop the biosensors have a high glucose affinity not suitable for detecting physiological concentrations of glucose.

Therefore, there is a need for pH-insensitive glucose biosensors with intrinsic signal transduction mechanisms to provide non-invasive methods of monitoring a wide range of glucose concentrations in vivo.

SUMMARY OF THE INVENTION

One aspect of the present invention encompasses a pH insensitive glucose indicator protein (GIP). The GIP comprises a glucose binding protein that changes conformation upon exposure to glucose, wherein said glucose binding protein comprises a phenyl alanine, valine, cysteine, threonine, or leucine at the amino acid position equivalent to amino acid position 16 of the mature E. Coli glucose/galactose binding protein. The GIP also comprises at least one pH insensitive fluorescence donor coupled to the glucose binding protein, and at least one pH insensitive fluorescence acceptor coupled to the glucose binding protein, wherein the conformational changes of the glucose binding protein upon binding or releasing glucose produces a detectable and reversible signal change.

Another aspect of the present invention encompasses a biosensor. The biosensor comprising at least two pH insensitive glucose indicator proteins (GIP), wherein each GIP comprises a glucose binding protein that changes conformation upon exposure to glucose, wherein said glucose binding protein comprises a phenyl alanine, valine, cysteine, threonine, or leucine at the amino acid position equivalent to amino acid position 16 of the mature E. Coli glucose/galactose binding protein; at least one pH insensitive fluorescence donor coupled to said glucose binding protein; and at least one pH insensitive fluorescence acceptor coupled to said glucose binding protein, wherein the conformational changes of the glucose binding protein upon binding or releasing glucose produces a detectable and reversible signal change.

Yet another aspect of the present invention encompasses an in vivo method for determining the concentration of glucose. The method comprises providing a biosensor comprising at least two pH insensitive glucose indicator proteins (GIP), wherein each GIP comprises: a glucose binding protein that changes conformation upon exposure to glucose, wherein said glucose binding protein comprises a phenyl alanine, valine, cysteine, threonine, or leucine at the amino acid position equivalent to amino acid position 16 of the mature E. Coli glucose/galactose binding protein; at least one pH insensitive fluorescence donor coupled to said glucose binding protein; and at least one pH insensitive fluorescence acceptor coupled to said glucose binding protein, wherein the conformational changes of the glucose binding protein upon binding or releasing glucose produces a detectable and reversible signal change. The method further comprises exposing said biosensor to varying glucose concentrations and measuring any change in the signal produced to determine the concentration of glucose.

Other aspects and iterations of the invention are described more thoroughly below.

REFERENCE TO COLOR FIGURES

The application file contains at least one photograph executed in color. Copies of this patent application publication with color photographs will be provided by the Office upon request and payment of the necessary fee.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts the amino acid sequence of glucose binding protein isolated from E. coli K12 (SEQ ID NO: 1). A leader peptide signal sequence and the leader peptide cleavage site are shown.

FIG. 2 depicts the SDS-PAGE analysis of GBP expression in E. coli. Lane M: protein molecular weight marker; Lane 1: control wild type E. coli lysate; Lane 2: lysate of E. coli expressing GBP; Lane 3: GBP purified by immobilized metal affinity purification.

FIG. 3 depicts the design of a glucose indicator protein (GIP) for sensing glucose based on FRET. (A) Diagram of the GIP structure showing how FRET between two green fluorescent proteins can measure glucose concentration. The GBP adopts an open form in the presence of glucose which triggers a conformation change causing the two GFP molecules to move apart, decreasing FRET. The black dot represents one molecule of glucose bound to the binding site of the GBP. GFPuv: green fluorescent protein with several mutations to enhance the xcitation by UV light. YFP: yellow fluorescent protein. GBP: glucose binding protein. FRET: fluorescence resonance energy transfer. (B) Domain structure of the GIP. The boundary region of amino acid sequence between the fused proteins was determined empirically to ensure the stable and efficient expression of GIP in E. coli. (C) Spectral overlap of GFPuv and YFP. The absorbance spectra are denoted by black lines, and emission spectra are denoted by gray lines.

FIG. 4 depicts the spectral properties of GIP and YFP-GBP. (A) The emission spectra of GIP at different wavelengths in the absence of glucose. Line a represents the emission spectrum of GIP excited at 395 nm, and line b represents the emission spectrum of GIP excited at 513 nm. (B) The excitation and emission spectra of YFP-GBP. Dotted line represents the emission spectrum, and the solid line represents the excitation spectrum.

FIG. 5 depicts the glucose affinity to GIP. Data points shown are the average of three experiments. Highly concentrated glucose was diluted into a buffer containing GIP, and the fluorescence intensity of GIP was measured at 527 nm when excited at 395 nm. Slit width was 5 nm. All measurements were conducted at room temperature.

FIG. 6 depicts a representation of a follow-fiber glucose biosensor constructed with GIP in which a GFP-based FRET signal transduction function was integrated for sensing glucose. A dialysis hollow fiber made from regenerated cellulose was used for including the protein. It was cut into a small fragment 1.5 cm in length. The dialysis hollow fiber had a molecular weight cutoff of 10 Kda allowing small molecules such as glucose to pass through the hollow fiber, while retaining GIP, which has a molecular weight of 86 Kda.

FIG. 7 depicts the response of a hollow fiber glucose biosensor constructed with GIP. Sugar-free PBS was used to provide a baseline for the sensor. Glucose (10 μM) was used in the experiment. The change of FRET was monitored by measuring the change in the fluorescence of GIP included in the hollow fiber. Excitation wavelength of 395 nm and emission wavelength of 527 nm were used. The data interval was 1 second. All experiments were carried out at room temperature.

FIG. 8 depicts a schematic diagram of the GIP for continuous glucose monitoring. The GIP was constructed by flanking a mutated GBP (GBPm) with two fluorescent proteins. The GBPm changes its conformation upon binding glucose, leading to a change in distance between ECFP and EYFP or a pH insensitive YFPi. This glucose binding-induced distance change alters the FRET that can be quantitatively measured by detecting by the change in the intensity ratio of fluorescence emitted by both YFP and ECFP.

FIG. 9 depicts the glucose titration curves of GIPs at vatying pH values. (A) pH-sensitive GIP. (B) pH-insensitive GIP. The experiments were performed at room temperature and glucose was titrated into the GIP solution. The slit width for both excitation and emission wavelengths were set at 10 nm. The fluorescent intensities of the ECFP and YFP were measured 10 min after the addition of glucose. The data shown are the mean values of three experiments and the error bars show the standard deviations (+/−SD).

FIGS. 10 (A) and (B) depict non-linear regression on glucose curves of mutated GIP: (A) GIP_(i)-Thr, GIP_(i)-Cys, GIP_(i)-Val and GIP_(i)-Ala; (B) GIP-Leu and GIP-Ala. Data shown are the mean values of three experiments and error bars represent the standard deviation.

FIG. 11 depicts experiments showing that mutated GIPs retain specificity for glucose and galactose. Purified GIP-Ala (A), GIP_(i)-Cys (B), and GIP_(i)-Thr (C) were incubated for 10 min in solutions containing three concentrations of various substrates. The ratio of emission intensity at 526 nm and 476 nm are plotted for each substrate tested. The data shown are the mean of three experiments and the error bars represent standard deviations.

FIG. 12 depicts glucose-induced FRET changes in C2C12 cells expressing GIP. (A) Pseudo-color images of the FRET ratio in a cell expressing GIP. Red color indicates high FRET and low glucose concentration, and green color indicates low FRET and high glucose concentration. (B) Alternating FRET ratio values during glucose addition and removal illustrating the stability and repeatability of the sensor inside the celluler environment. The FRET ratio was measured at six second intervals during the perfusion of bath solutions containing 10 mM glucose or no glucose.

FIG. 13 depicts measurements and visualization of glucose within living cells through fluorescence lifetime microscopy imaging. (A) Time-lapse Intracelluar glucose response imaged in C2C12 cells using AcGFP-GBPval-mCherry protein sensor. Upward arrows indicate external glucose addition (intracellular uptake) and downward arrows indicate the clearance of glucose from the cells. (B) Representative pseudocolor FRET ratio images (selected between time point A and B indicated in the image) from an individual C2C12 cell showing the increase intracellular glucose concentration. Typical glucose uptake and clearance curves were fitted to first order exponential and the time constant (tau) values were calculated.

FIG. 14 depicts the determination of lifetime of GIP in transfected murine myoblast C2C12 cells. (A) Phase contrast micrograph and (B) fluorescence lifetime microscopy image of C2C12 cells transfected with GIPval. The images were taken 48 h post transfection. (C) Phase and modulation lifetimes of ECFP.

FIG. 15 depicts the determination of phase and modulation lifetime of AcGFP in living cells. (A) A phase contrast micrograph and (B) fluorescence lifetime microscopy image of pcDNA3.1-AcGFP-GBPval-mCherry transfected C2C12 cells. (C) phase and modulation lifetime of AcGFP.

FIG. 16 depicts the determination of photo bleaching of AcGFP in transfected C2C12 cells. (A) Photo bleaching of AcGFP during the fluorescence lifetime microscopy measurement and (B) time course of lifetime of AcGFP during fluorescence lifetime microscopy imaging.

FIG. 17 depicts C2C12 cells expressing GIP imaged in presence of 20 mM extracellular glucose. (A) Fluorescence intensity image and (B) a pseudo colored lifetime image. (C) The average lifetime intensity for two ROIs. (D) The total fluorescence intensity and donor lifetime for two ROIs.

FIG. 18 depicts C2C12 cells showing the expression of (A) AcGFP-GBPcys-mCherry. (B) A typical fluorescent lifetime map of the cell, which is merged with (C) fluorescent intensity image are also shown.

FIG. 19 depicts raw data from the frequency domain lifetime imaging system for the cell showed in FIG. 18.

FIG. 20 depicts the uptake of glucose in two segments of a cell. The tau values were found to be about 45 s.

FIG. 21 depicts the clearance of glucose.

FIG. 22 depicts the change in fluorescence intensity in the area of the cell selected for the glucose monitoring.

FIG. 23 C2C12 cells grown under physiological glucose concentration visualized with GIP. (A) Phase contrast micrograph, (B) FRET-FLIM image, and (C) overlapped cellular images of GIP expression plasmids transfected C2C12 cells. The images were taken 48 h post transfection.

FIG. 24 depicts glucose response in the C2C12 cells maintained at physiological glucose concentration, visualized by the FRET-FLIM measurement. Three traces are from 3 different cells shown in FIG. 23.

FIG. 25 depicts single exponential fit for the glucose uptake in C2Cl2 cells maintained in physiological glucose levels (5 mM).

FIG. 26 depicts a sequence encoding sequence of GIP_(i)-Thr (SEQ ID NO: 2).

FIG. 27 depicts a sequence encoding sequence of GIP_(i)-Cys (SEQ ID NO: 3).

FIG. 28 depicts a sequence encoding sequence of GIP_(i)-Leu (SEQ ID NO: 4).

FIG. 29 depicts a sequence encoding sequence of GIP_(i)-Val (SEQ ID NO: 5).

FIG. 30 depicts a sequence encoding sequence of a plasmid encoding the GIP: AcGFP-GBPval-mCherry (SEQ ID NO: 6).

FIG. 31 depicts a sequence encoding sequence of a plasmid encoding AcGFP-GBPcys-mCherry (SEQ ID NO: 7).

FIG. 32 depicts a sequence encoding sequence of a plasmid encoding AcGFP-GBPthr-mCherry (SEQ ID NO: 8).

FIG. 33 depicts a sequence encoding sequence of a plasmid encoding the AcGFP-GBPthrDelta12-mCherry (SEQ ID NO: 9).

DETAILED DESCRIPTION OF THE INVENTION

A glucose indicator protein (GIP) has been developed. Significantly, the GIP is insensitive to in vivo pH changes. Additionally, the GIP is sensitive to a wide range of glucose concentrations. Advantageously, a GIP of the invention may be used to measure in vivo glucose concentrations.

I. Glucose Indicator Protein

One aspect of the present invention encompasses a glucose indicator protein (GIP). Generally speaking, a GIP comprises a glucose binding protein, a fluorescence donor, and a fluorescence acceptor. Conformational changes in the glucose binding protein caused by glucose binding results in a change in fluorescent resonance energy between the fluorescent acceptor and donor, resulting in a detectable signal. Components of the GIP are discussed in more detail below.

(a) Glucose Binding Protein

A GIP of the invention comprises a glucose binding protein. As used herein, “glucose binding protein” refers to a protein that changes conformation upon glucose binding. A glucose binding protein should be capable of reversibly binding glucose. In some embodiments, a glucose binding protein may be a glucose/galactose binding protein (GGBP) belonging to the family of bacterial periplasmic-binding proteins. In an exemplary embodiment, a glucose binding protein is the glucose/galactose binding protein of E. coli. In a further exemplary embodiment, a glucose binding protein is the GGBP of E. coli K12 encoded by mgIB. The E. coli GGBP comprises a leader peptide signal sequence that is cleaved upon maturation of the protein (see FIG. 1A). In some embodiments, the glucose binding protein of the invention comprises the full length GGBP (including the leader sequence). In other embodiments, the glucose binding protein of the invention comprises the mature GGBP (without the leader sequence).

In some embodiments, the invention comprises a wild type GGBP. In other embodiments, the invention comprises a mutated GGBP. A mutated GGBP refers to a binding protein that contains one or more amino acids that have been substituted for, deleted from, or added to the amino acids present in the wild type protein. In one embodiment, a mutation may affect the pH sensitivity of the binding protein. In another embodiment, a mutation may be made in a residue of the binding protein that contacts glucose, or that is involved in glucose binding. Stated another way, a mutation may alter the affinity of the binding protein for glucose, or alter the Kd. In some embodiments, the amino acid at position 16 of the mature E. coli GGBP, or the equivalent amino acid position in another GGBP, may be mutated. In other embodiments, the amino acid at position 183 of the mature E. coli GGBP, or the equivalent amino acid position in another GGBP, may be mutated. By way of non-limiting example, the phenylalanine at position 16 may be substituted by valine, cysteine, threonine, or leucine. In one embodiment, a glucose binding protein comprises the E. Coli K12 GGBP with a phenylalanine at position 16. In another embodiment, a glucose binding protein comprises the E. Coli K12 GGBP with a valine at position 16. In yet another embodiments, a glucose binding protein comprises the E. Coli K12 GGBP with a cysteine at position 16. In still another embodiment, a glucose binding protein comprises the E. Coli K12 GGBP with a threonine at position 16. In a further embodiment, a glucose binding protein comprises the E. Coli K12 GGBP with a leucine at position 16.

In an alternative embodiment, a glucose binding protein may comprise an E. coli GGBP with an N-terminus deletion. The deletion may comprise about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or more than 15 amino acids.

In some embodiments of the invention, the Kd value for glucose and the glucose binding protein are from about 0.0001 mM to about 20 mM. For instance, the Kd value may be about 0.0001, 0.001, 0.01, 0.1, 1, 10, or 20 mM. Alternatively, the Kd value may be between about 0.0001 mM and about 0.001 mM, between about 0.001 mM and about 0.01 mM, between about 0.01 mM and about 0.1 mM, between about 0.1 mM and about 1 mM, between about 1 mM and about 10 mM, or between about 10 mM and about 20 mM. In an exemplary embodiment, the Kd value ranges from about 0.01 mM to about 10 mM. In another exemplary embodiment, the Kd value may be about 0.01, 0.05, 0.1, 0.15, 0.2. 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0. 9.5, or 10.0 mM.

(b) Fluorescent Donor and Acceptor

The glucose indicator protein comprises at least one fluorescence donor and at least one fluorescence acceptor coupled to the glucose binding protein. Typically, the fluorescence donor and the fluorescence acceptor may be a fluorescent protein, a quantum dot, or a dye. In preferred embodiments, the fluorescence donors and acceptors are fluorescent proteins. Non-limiting examples of fluorescent proteins may include fluorescent proteins isolated from Aequorea victoria, Aequorea coerulescens, Renilla reniformis, Zoanthus sp., Anemonia majano, Anemonia sulcata, Heteractis crispa, Discosoma striata, Clavularia sp. and Phialidium gregarium. Further non-limiting examples may include variants of wild-type fluorescent proteins that possess improved fluorescence, improved stability, different physiological requirements, or altered excitation emission spectra. Suitable examples are known in the art. In one embodiment, the fluorescence donor or acceptor may be Aequoria victoria green fluorescent protein (GFP), Aequorea coerulescens GFP (AcGFP), enhanced green fluorescent protein (EGFP), GFPuv, blue fluorescent protein (BFP), cyan fluorescent protein (CFP), enhanced cyan fluorescent protein (ECFP), yellow fluorescent protein (YFP), the pH insensitive variant of YFP (YFP_(i)), red fluorescent protein (RFP), amFP486, cFP484, drFP583, and mCherry.

In preferred embodiments, the fluorescence donor and the fluorescence acceptor of the invention are pH insensitive. Insensitivity to pH may be a natural characteristic of the wild type form of the fluorescent protein, or may be the result of specific mutations of the fluorescent protein. Non-limiting examples of pH insensitive fluorescence donors and fluorescence acceptors may include YFPi, CFP, AcGFP and mCherry.

More than one fluorescent donor or fluorescent acceptor may be coupled to the glucose binding protein. For instance, in some embodiments, three fluorescence donors and one fluorescence acceptor may be coupled to the glucose binding protein. In other embodiments, one fluorescence donor and three fluorescence acceptors may be coupled to the glucose binding protein. In certain embodiments, three fluorescence donors and two fluorescence acceptors may be coupled to the glucose binding protein. In one embodiment, two fluorescence donors and three fluorescence acceptors may be coupled to the glucose binding protein. In another embodiment, two fluorescence donors and one fluorescence acceptor may be coupled to the glucose binding protein. In yet another embodiment, one fluorescence donor and two fluorescence acceptors may be coupled to the glucose binding protein. In still other embodiments, two fluorescence donors and two fluorescence acceptors may be coupled to the glucose binding protein. In an alternative embodiment, one fluorescence donor and one fluorescence acceptor may be coupled to the glucose binding protein.

One or more fluorescence donors or fluorescence acceptors may either be coupled to the N-terminus or the C-terminus of the glucose binding protein. In some embodiments, one or more fluorescence donors are coupled to the C-terminus of the glucose binding protein, and one or more fluorescence acceptors are coupled to the N-terminus of the glucose binding protein. In other embodiments, one or more fluorescence donors are coupled to the N-terminus of the glucose binding protein, and one or more fluorescence acceptors are coupled to the C-terminus of the glucose binding protein.

Glucose binding effects a change in conformation of the glucose binding protein that alters the relative position of the fluorescence donor and fluorescence acceptor. This results in increased separation of the donor and acceptor, and results in a change in FRET signal. For excitation transfer from donor to acceptor to occur, the absorption spectrum of the acceptor must overlap the fluorescence emission spectrum of the donor. As a result, the selection of a donor limits the selection of an acceptor, and similarly, the selection of an acceptor limits the selection of a donor. Selection of an appropriate donor and acceptor pairs may be performed by one of skill in the art. Non-limiting examples of suitable pairs of fluorescence donors and fluorescence acceptors capable of producing a FRET signal may include BFP and GFP, GFPuv and YFP, CFP and YFP, ECFP and YFPi, AcGFP and mCherry. In one embodiment, a suitable pair comprises ECFP and YFPi. In another embodiment, a suitable pair comprises AcGFP and mCherry.

(c) Isolation Tags

The glucose indicator protein of the invention may also comprise a tag to simplify isolation. In some embodiments, the glucose indicator protein may be operably linked to an antibody epitope such that the glucose indicator protein is epitope tagged. Non-limiting examples of suitable antibody epitope tags may include AcV5, AU1, AU5, E, ECS, E2, FLAG, Glu-Glu, HSV, KT3, myc, S, S1, T7, V5, VSV-G, TAP tag, DDDDK, and 6×His. In one embodiment, the epitope tag may be a 6×His tag. The location of the antibody epitope tag may vary depending upon the embodiment. For instance, in one embodiment, the epitope tag may be located at the carboxyl terminus of the glucose binding protein. In another embodiment, the epitope tag may be located at the carboxyl terminus of the fluorescence donor. In yet another embodiment, the epitope tag may be located at the amino terminus of the fluorescence donor. In still another embodiment, the epitope tag may be located at the carboxyl terminus of the fluorescence acceptor. In still yet another embodiment, the epitope tag may be located at the amino terminus of the fluorescence acceptor. In certain embodiments, the epitope tag may be located at the amino terminus of the glucose binding protein.

(d) Coupling

The fluorescent acceptor and donor may be coupled to the glucose binding protein through standard molecular biology procedures well known to those of skill in the art. In one embodiment, a fluorescent acceptor or donor may be coupled to the glucose binding protein through a linker. The length and flexibility of the linker moiety is generally chosen to optimize both FRET and the kinetics and specificity of conformational changes induced by glucose binding. As is commonly recognized in the art, linkers may be empirically determined. In some embodiments, the linker is a peptide moiety. In other embodiments, the linker is a peptide from about one to about 30 amino acid residues in length. In still other embodiments, the linker is a peptide from about two to about 15 amino acid residues. For instance, the linker may be about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 amino acid residues. In one embodiment, the linker between a fluorescence donor and a glucose binding protein may comprise --Thr-Ser--. In another embodiment, the linker between a fluorescence acceptor and a glucose binding protein may comprise --Gly-Thr--. In yet another embodiment, the linker between a fluorescence donor and a glucose binding protein may comprise --Thr-Ser-- and the linker between a fluorescence acceptor and a glucose binding protein may comprise --Gly-Thr--.

(e) Specific GIPs

In some embodiments, a GIP of the invention comprises a GBP, donor and acceptor combination listed in Table A below. In each case, the GBP listed in Table A is the E. coli GGBP.

TABLE A Position 16 of the GBP Donor Acceptor phenylalanine ECFP YFPi valine ECFP YFPi cysteine ECFP YFPi threonine ECFP YFPi leucine ECFP YFPi alanine ECFP YFPi phenylalanine AcGFP mCherry valine AcGFP mCherry cysteine AcGFP mCherry threonine AcGFP mCherry leucine AcGFP mCherry alanine AcGFP mCherry phenylalanine BFP GFP valine BFP GFP cysteine BFP GFP threonine BFP GFP leucine BFP GFP alanine BFP GFP phenylalanine GFPuv YFP valine GFPuv YFP cysteine GFPuv YFP threonine GFPuv YFP leucine GFPuv YFP alanine GFPuv YFP phenylalanine CFP YFP valine CFP YFP cysteine CFP YFP threonine CFP YFP leucine CFP YFP alanine CFP YFP

(f) Methods of Making a GIP

A GIP of the invention may be synthetically produced, or alternatively, may be expressed in a cell. Methods for expressing a protein in a cell involve the use of molecular cloning techniques well known in the art. Such techniques are described in Sambrook et al., Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., (1989) and Current Protocols in Molecular Biology, F. M. Ausubel et al., eds.

In one embodiment, a GIP may be expressed in a prokaryotic cell. Suitable prokaryotic cells are known in the art. For instance, a GIP may be expressed in E. coli. In another embodiment, a GIP may be expressed in a eukaryotic cell. Suitable eukaryotic cells are known in the art. In some embodiments, the eukaryotic cell may be of human origin. In other embodiments, the cell may be of animal origin. In certain embodiments, the cell may be from an established or a primary cell line. Such cell lines may be isolated, characterized, and expanded using standard techniques known to individuals skilled in the art. The cell line may be adherent or non-adherent, or the cell line may be grown under conditions that encourage adherent, non-adherent or organotypic growth using standard techniques known to individuals skilled in the art. The cell line may be contact inhibited or non-contact inhibited. The cell line may be derived from the digestive system, the skeletal system, the muscular system, the nervous system, the endocrine system, the respiratory system, the circulatory system, the reproductive system, the integumentary system, the lymphatic system, or the urinary system. In some embodiments, the cell line may be derived from epithelial tissue. In other embodiments, the cell line may be derived from nervous tissue. In yet other embodiments, the cell line may be derived from connective tissue. In preferred embodiments, the cell line may be derived from muscle tissue. A non-limiting list of cell lines commonly used in the laboratory setting may include Chinese hamster ovary cells (CHO), vero African green monkey kidney cells, C2C12 mouse myoblast, or HeLa human cervical cancer cells. In preferred embodiments, the cell line may be C2C12 mouse myoblast.

In each of the above embodiments, the GIP may be expressed using a plasmid. Suitable plasmids are known in the art.

Isolating and purifying a GIP expressed in a cell may be performed using techniques generally known in the art. For example, affinity purification may be used to isolate a GIP. Alternatively, column chromatography techniques, precipitation protocols and/or other methods for separating proteins may also be used. (see, e.g., Scopes, Protein Purification: Principles and Practice (1982); U.S. Pat. No. 4,673,641; Ausubel et al., supra; and Sambrook et al., supra; and Leonard et al., J. Biol. Chem. 265:10373-10382 (1990).

(g) Encapsulation of a GIP of the Invention

In certain embodiments, a GIP of the invention may be encapsulated in a semi permeable membrane. See, for instance, FIG. 6. Typically, the semi permeable membrane may be selected so as to be permeable to the passage of glucose, but impermeable to the passage of blood clots, cells, and proteins. In some embodiments, the membrane may be selected so as to exclude most materials having a molecular weight of about 5,000 daltons to about 20,000 daltons. In other embodiments, the membrane may be selected so as to exclude most materials having a molecular weight greater than about 10,000 daltons. Non-limiting examples of such enclosures and semi permeable membranes are discussed in Heller, U.S. Pat. No. 5,593,852, Wilkins, U.S. Pat. No. 5,431,160, Hogen Esch, U.S. Pat. No. 5,372,133, Zier, U.S. Pat. No. 4,919,141, and Gough, U.S. Pat. No. 4,703,756, all hereby incorporated in full by reference.

If a semi-permeable membrane encompassing a GIP is to be implanted in a mammalian body, the semi permeable membrane is preferably an inert, nontoxic material. Non-limiting examples of suitable semi permeable material may include cellulose acetate, cellulose, methyl cellulose, polyvinyl alcohol, polyurethane, and combinations thereof. In one embodiment, a GIP may be encapsulated in a cellulose hollow fiber semi permeable membrane.

II. Biosensor Comprising at Least One GIP

Another aspect of the invention encompasses a biosensor comprising at least one GIP. In one embodiment, the biosensor may comprise one GIP. In another embodiment, the biosensor may comprise two GIPs. In yet another embodiment, the biosensor may comprise three GIPs. In still another embodiment, the biosensor may comprise four GIPs. In a further embodiment, the biosensor may comprise five GIPs. In yet a further embodiment, the biosensor may comprise six GIPs. In an alternative embodiment, the biosensor may comprise seven GIPs. In another alternative embodiment, the biosensor may comprise eight GIPs. In still another alternative embodiment, the biosensor may comprise nine GIPs. In some embodiments, the biosensor may comprise ten GIPs. In other embodiments, the biosensor may comprise more than ten GIPs.

Suitable GIPs include those detailed in section I above. In an exemplary embodiment, suitable GIPs include those detailed in Table A above. In some embodiments, a biosensor of the invention may be encapsulated as detailed in section 1(g) above.

III. Method for In Vivo Detection of Glucose

Yet another aspect of the present invention encompasses a method for detecting and measuring in vivo glucose concentration. Generally, such a method comprises contacting a GIP or biosensor of the invention with glucose in vivo, and detecting the change in FRET, wherein the change in FRET is correlated with a change in glucose concentration.

In some embodiments, a GIP or biosensor of the invention may be exposed to glucose when expressed in a cell. Suitable cells may be grown, sub-cultured, stored and manipulated using standard techniques known to individuals skilled in the art. Generally speaking, the cells should be maintained in effective conditions, meaning conditions that support cell growth/proliferation if essentially no other regulatory compounds are present that would interfere with cell growth/proliferation. The number of cells seeded for the assay will vary with growth conditions as cell growth can be influenced by many factors such as the size of the container used, the rate of growth of the cells used, and composition of the media used to grow the cells.

In other embodiments, a GIP or biosensor of the invention may be exposed to glucose by directly contacting a GIP or biosensor with a solution containing glucose. In an alternative embodiment, a GIP or a biosensor may be exposed to glucose by encapsulating a GIP or biosensor in a semi permeable membrane and exposing the membrane to a solution comprising glucose. Such a solution may comprise blood or interstitial fluid. For more details, see the Examples.

After a GIP or biosensor of the invention is contacted with glucose, the change in FRET may be measured. Methods of measuring FRET are well known in the art and are described in, e.g., “Review in Fluorescence” (2004) Chris D. Geddes (Editor), Joseph R. Lakowicz (Editor), which is hereby incorporated by reference in its entirety. Non-limiting examples of methods to measure FRET may include luminescence spectroscopy, fluorescence microscopy, fluorescent lifetime imaging microscopy (FLIM), and photobleaching FRET. In an exemplary embodiment, the change in FRET may be monitored by FLIM, as detailed in the examples.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which this invention belongs. The following references provide one of skill with a general definition of many of the terms used in this invention: Singleton et al., Dictionary of Microbiology and Molecular Biology (2nd ed. 1994); The Cambridge Dictionary of Science and Technology (Walker ed., 1988); The Glossary of Genetics, 5th Ed., R. Rieger et al. (eds.), Springer Verlag (1991); and Hale & Marham, The Harper Collins Dictionary of Biology (1991). As used herein, the following terms have the meanings ascribed to them unless specified otherwise.

The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

EXAMPLES

The following examples illustrate various iterations of the invention.

Example 1 Intracellular Expression of Glucose Binding Protein (GBP)

A 32 kDa periplasmic glucose binding protein (GBP) encoded by the mglB gene that serves as a glucose transporter in Escherichia coli K12 was selected for this invention (FIG. 1). The initial 23 N-terminal amino acid sequence serves as a leader peptide signal that directs the polypeptide to the bacterial periplasmic space. To ensure the intracellular expression of GBP, the leader peptide signal sequence was deleted from the cDNA of GBP. To facilitate purification, a polyhistidine affinity tag (His)6 was fused to the C-terminus of the GBP.

The protein possesses two distinct helical structural domains, each organized in an a/b folding motif involving the glucose binding region. X-ray structural analysis suggests that the NH₂-terminal and COOH-terminal domains are composed of a core of parallel β-sheet flanked by two layers of α-helices.

The expression of intracellular GBP in E. coli DH5α transformed with the GBP expression plasmid pTAGBP was determined using SDS-PAGE analysis (FIG. 2) after induction with IPTG. Glucose (4 g/L) was added to the LB media to stabilize the expressed GBP protein. Wild type E. coli DH5α lysate served as a control for the assay. The expression of GBP was confirmed in the cell lysate of E. coli/DH5α/pTAGBP (lane 2 in FIG. 2) but not in the wild type cell lysate (lane 1 in FIG. 2). The GBP band migrated at a 34 kDa position, which is the expected size of a (His)6-tagged GBP. Clearly, the truncated GBP was efficiently expressed in E. coli. Moreover, SDS-PAGE assays showed that the expression of the GBP started from 2 hr post induction and reached a maximum at ˜4 to 6 h post induction (data not shown). The expressed GBP could be concentrated nearly 15-fold by an immobilized metal affinity purification (IMAC) column. The purity of GBP eluted from the IMAC column was very high (lane 3 in FIG. 2).

These data demonstrated that GBP could be efficiently expressed intracellularly by the deletion of the leader peptide signal sequence. Previous data in the literature showed low levels of protein expression when confined to the periplasmic space. Thus the methods developed here for the intracellular expression of GBP are of critical importance for large-scale production of GBP for use in glucose biosensors.

Example 2 Construction of the Glucose Indicator Protein (GIP)

The tertiary structure of GBP has two similar N- and C-terminal domains. Both the N- and C-terminal domains are composed of a core of parallel β-sheets flanked by two layers of α-helices. The cleft between the two domains has been identified as the glucose binding site. A calcium binding loop is present in the C-terminal domain. When glucose binds to GBP, the protein undergoes a conformational change, resulting in an alteration of the distance between the C- and N-terminal domains. To achieve a measurable signal upon glucose binding, fluorescent reporter proteins were fused to GBP in such a manner that the spatial separation between the fluorescent moieties changes when the ligand binds (FIG. 3A). As the diameter of GBP is about 50 Angstroms, it will provide a separation between the two reporter proteins and allows the fluorescence energy transfer technique to be used for glucose.

To construct a GIP, two fluorescent proteins were fused, one to each end of the GBP. The amino acid sequences of the boundary region between fusion proteins were determined empirically to achieve a correct and stable folding of the fusion protein. In essence, the GIP has four domains (FIG. 3B). The central two domains (of GBP) involve the glucose binding region that undergoes a change in conformation upon glucose binding. The other two peripheral domains are the added green fluorescent proteins. GFPuv, which was fused to the C-terminus of GBP, served as a donor to transfer the fluorescence energy to the receptor YFP that was fused to the other terminus, the N-terminus, of GBP.

GFPuv has a maximum excitation wavelength at 395 nm and a maximum emission wavelength at 510 nm, whereas YFP has a maximum excitation wavelength at 513 nm and a maximum emission wavelength at 527 nm. Thus high energy transfer efficiency can be achieved as a result of the large spectral overlap between the emission spectra of GFPuv and the absorption spectra of YFP (FIG. 3C). When glucose binds to the protein, rearrangement of the flap region located on one side of the hinge β-sheet of GBP occurs. The change in conformation of the GBP moiety upon the binding of glucose, in turn, alters the relative position of GFPuv (donor) and YFP (acceptor) resulting in increased separation and reduction of FRET (FIG. 3A).

Example 3 Fluorescent Resonance Energy Transfer using GIP

The GIP described in Example 2 was expressed in E. coli, purified, and dialyzed against sugar-free binding buffer. Sugar-free proteins were concentrated by ultrafiltration and stored at 4° C. in the dark until use. To determine the glucose response of GIP, the spectra of GIP were first characterized by scanning its emission and/or excitation wavelength using a luminescence spectrometer. The emission of the GIP showed two peaks: one at 510 nm and another at 527 nm when excited at 395 nm (curve a in FIG. 4A). The appearance of an emission peak at 527 nm indicates that some energy transfer occurs from the donor GFPuv to the acceptor YFP in the absence of glucose. The excitation spectra of GIP measured at the emission wavelength of 527 nm revealed three excitation peaks, that is, 395, 488, and 513 nm. It is known that both GFPuv and YFP have a small excitation spectrum peak at 488 nm. Excitation of GIP at 513 nm produced an emission peak at 527 nm (curve b in FIG. 4A), which is the typical emission peak of YFP when excited at 513 nm. This indicated that the fusion of GFPuv-GBP to the C-terminus of YFP does not change the fluorescence spectrum of YFP.

No emission of YFP-GBP could be detected when excited at 395 nm, although it had an emission peak at 527 nm when excited at 513 nm (FIG. 4B). Clearly, in the absence of glucose, the emission of GIP at 527 nm when excited at 395 nm is caused by fluorescence energy transfer. This result is consistent with the X-ray studies on the structure of the glucose binding protein, which reveals that it can adopt at least two different and stable conformations. The protein is able to adopt a closed form in the absence of ligand (glucose/galactose), which places the two terminal domains close to each other. The space between the two domains located in sites 15 and 152 (glucose binding pocket) of GBP shortens the distance between two fluorescent proteins located at N- and C-termini of GBP. On the other hand, the two domains can be relatively far apart in the presence of bound glucose, resulting in a reduction of the fluorescence intensity at 527 nm when excited at 395 nm.

Example 4 Binding Isotherm of GIP for Glucose

The binding affinity of GIP for glucose was determined by the fluorescence energy transfer method (FIG. 5). GIP was dissolved in the binding buffer and highly concentrated glucose was titrated into the solution. The total volume of the glucose added to the protein throughout the entire procedure was <5 ml. The conformational change induced by glucose binding was determined by measuring the fluorescence intensity of GIP using an excitation wavelength of 395 nm and an emission wavelength of 527 nm. A decrease in the fluorescence intensity from GIP was observed with the addition of glucose, indicating the reduction of FRET between GFPuv and YFP. Moreover, the decrease in fluorescence intensity was linearly related to glucose concentration up to 20 mM. The GIP binding site appeared to become saturated when the glucose concentration was raised above 20 mM, consistent with the reported binding affinity of GBP. Therefore, the apparent Km for GIP is ˜5 mM.

Example 5 Glucose Microsensor using GIP

Finally, a glucose microsensor using GIP was designed as shown in FIG. 6. In this prototype glucose biosensor, GIP was sealed into a dialysis hollow fiber that had an internal diameter of 190 mm, wall thickness of 20 mm, and cutoff molecular weight of 10 kDa. Thus, the protein is retained inside the hollow fiber but small molecules such as glucose can permeate through the dialysis fiber and reversibly bind to GIP, generating a conformational change-induced change in FRET. The hollow fiber microsensor was placed into a quartz cuvette that had an inner cylinder hole with a 7 ml sampling volume. The segment was fixed inside the cell by pushing it down with the inflow cover of the flow-through cell unit. The whole unit was then set up into a luminescence spectrometer. A sugar-free PBS buffer containing 1 mM CaCl₂ was gravity-perfused into the flowthrough unit and a baseline for the sensor was produced. A decrease in fluorescence intensity was observed the when the glucose (10 mM) solution flowed past the sensor, indicating the reduction of FRET between the two fluorescent proteins GFPuv and YFP (FIG. 7). Fluorescence intensity recovered when PBS buffer was circulated past the sensor, which removed the glucose (by dialysis) from the encapsulated GIP. The response time was ˜100 s. Therefore, it can be concluded that the kinetics of binding and conformational change must be faster than 100 seconds because there are some delays in the exchange of the perfusing solution and the diffusional lag due to the barrier properties of the dialysis tube itself. As is apparent from the data in FIG. 7, the sensor response to glucose was fairly reproducible and reversible over this time period.

Example 6 Development of pH Insensitive GIP

As it is evident from the data in FIG. 9A, measurement of glucose using the GIP constructed in Example 2 was sensitive to pH. To alleviate the pH effect on the conformational change-induced FRET upon the glucose binding to GIP, a pH insensitive GIP was constructed by replacing pH sensitive components of GIP with pH insensitive components. In this experiment, a mutant form of GBP (GBPm) where Phe 16 is replaced with Ala (See Example 7 below) was used. The pK_(d) of GBPm was 589 mM for glucose and pK_(d) of 848 mM for galactose. Thus the GIP constructed with this mutated glucose binding protein should be able to measure a glucose concentration up to 5 mM. GBPm was flanked at its N and C termini with an ECFP (enhanced cyan fluorescent protein), and a pH insensitive YFPi (yellow fluorescent protein), respectively (FIG. 8). The resulting GIP was designated C_(o)Y_(i). In a control construct, enhanced YFP (EYFP) was used instead of YFPi to produce the GIP refered to as C_(o)Y_(o).

Sensitivity of the resulting GIPs to pH was then tested in a glucose binding affinity test (FIG. 9). To establish a calibration curve of C_(o)Y_(i), the highly concentrated glucose was added directly to the protein solution and the change of fluorescence intensity ratios of YFPi over ECFP was measured. This experiment indicates that 10 min was sufficient for glucose binding to the GIPs as the fluorescence intensity ratio tended to be stable after 10 min incubation at room temperature (data not shown). These observations are consistent with our previous studies on the glucose binding to GIPs. As previously reported, the conformational change-induced FRET can be determined by fitting the substrate titration curves to the equation for the binding of a ligand to a protein.

$\begin{matrix} {\frac{R_{\max} - R}{R_{\max} - R_{\min\bullet}} = \frac{{n\lbrack S\rbrack}^{n}}{K_{d} + \lbrack S\rbrack^{n}}} & (1) \end{matrix}$

where [S] is the glucose concentration, Kd is the apparent dissociation constant that corresponds to a glucose concentration yielding an R (fluorescent intensity ratio of YFP to ECFP) midway between R_(max) and R_(min), n is the Hill coefficient. The (R_(max)−R)/(R_(max)−R_(min)) is defined as a saturation of the FRET. As shown in FIG. 9A, the glucose titration curve of the pH sensitive C_(o)Y_(o) at pH=6.2 was almost the same as that determined at pH=7.3. Nevertheless, the response of C_(o)Y_(o) to glucose fluctuated wildly when pH dropped to 5.2 due to the instability of EYFP under an acidic environment. In contrast, C_(o)Y_(i) exhibited high stability under an acidic environment. No significant deviation in the glucose titration curves was found when exposing the C_(o)Y_(i) to an acidic environment. The glucose titration curves of C_(o)Y_(i) obtained at pH=7.3, 6.2, and 5.3 were almost the same, showing much improvement in reporting a quantitatable and consistent glucose measurement in a varying pH environment. Therefore, C_(o)Y_(i) is an ideal GIP for intracellular glucose studies where the pH values could vary significantly.

By using a pH-stable YFP to construct the glucose indicator C_(o)Y_(i), we were able to show that the C_(o)Y_(i) was more tolerant to the acidic environment. The resulting conclusion from this work is that a pH-insensitive glucose indicating protein can be engineered through flanking a glucose binding protein with pH insensitive fluorescent proteins such as YFPi used in this work. This protein allows for the quantitation of glucose concentration despite a changing pH environment. The conception and production of a pH-insensitive GIP is a great tool for use in varying pH environments. An exemplary application is the measurement of intracellular glucose. The use of this GIP as a noninvasive glucose measurement instrument will work well as interstitial glucose levels tend to follow blood glucose concentrations within a 3 to 5 minute time frame.

Example 7 Construction of an Array of GIPs with the Ability of Sensing a Wide Range of Glucose Concentrations

Wild-type GBP has phenylalanine at the 16th amino acid residue and has a carbohydrate ligand binding pocket. Crystallographic analysis of wild-type GBP reveals that the D-glucose/D-galactose is sandwiched in the GBP binding pocket by facing Phe16 and Trp183 of GBP through extensive van der Waals interactions, indicating the super-positioning effect of the 16th and 183th amino acid in the sugar binding site. In an attempt to alter the affinity of GBP and to improve the glucose monitoring range of GIPs constructed with GBP for real-time glucose detection, the effect of point mutation at the 16th amino acid residue of the GBP on its glucose binding affinity was examined. In brief, Phe16 of GBP in C_(o)Y_(i) constructed in Example 6 was replaced with either valine, cysteine, threonine, or leucine through site-directed mutagenesis. The resultant GIPs were designated as GIP_(i)-Val, GIP_(i)-Cys, GIP_(i)-Thr, GIP_(i)-Leu and GIP_(i)-Ala. A protein variant with a single amino acid deletion of the 16th amino acid of GBP was constructed and assigned as GIPi-Null for control measurements. Similar point mutations, in addition to a Phe16 to Leu substitution, were also made to the original GIP constructed in Example 2.

Titration of the purified GIPs presented a glucose dose-dependent decrease in FRET (FIG. 10). The binding affinities of mutated GIP were determined using non-linear regression and the properties of mutated GIP nanosensors were listed in Table B. The binding affinities of GIP_(i)-Cys and GIP_(i)-Val were 0.13 mM and 0.33 mM, respectively, which decreased the affinity of the sensor for glucose as compared with wild-type phe16 of GBP. The threonine substitution had an affinity of 7.9 mM, and therefore, a greatly decreased binding affinity for glucose. As two references, the binding affinities of GIPi-Ala and GIP-Leu were determined as 0.554 mM and 0.605 mM, respectively. For GIPi-Thr, where threonine substitutes the Phe at amino acid position 16, the glucose sensing range is spanned from 1 mM to 32 mM (FIG. 10A). The substitution of cysteine and valine manages the glucose sensing range from the 0.01 mM to 0.8 mM, and the 0.01 mM to 1.0 mM, respectively. Leucine mutation from phenylalanine presented 5-fold increase of Kd value with sensing range from 0.004 mM to 0.4 mM. It is worth to note that the average value of maximum number of binding sites was calculated as 0.984, indicating the one-site specific binding (Table B).

TABLE B Binding properties of the glucose nanosensors ^(a) Nanosensor K_(d) (mM) for glucose ^(b) Std. Error of the mean R² GIPi-Cys 0.131 0.026 0.920 GIPi-Thr 7.859 1.612 0.927 GIPi-Val 0.331 0.085 0.846 GIPi-Ala 0.554 0.084 0.942 GIP-Leu 0.026 0.005 0.973 GIP-Ala 0.605 0.077 0.959 ^(a) Determinations are averages of data collected from three measurments. ^(b) Calculation of the binding affinity was performed using steady-state intensity data fit to one-site saturation binding isotherm with non-linear regression.

Example 8 Specificity of GBP Mutants to Glucose

Monitoring glucose concentrations in living cells requires sensors possessing a high specificity to glucose. Thus it was next investigated whether the mutations of GBP described in Example 7 would lead to binding other carbohydrates. To assess specificity of the mutated GIP biosensors for glucose, a panel of related sugars was used to examine the specificity of the mutated GIPs. Proteins were purified from E. coli and analyzed for binding to galactose, fructose, ribose, arabinose, sorbitol, melibiose, xylose, and trehalose, and compared with binding of glucose by measuring the ratio of emission intensity at 526 nm and 476 nm. The GIP_(i) mutants demonstrated the exact same specificity for all the sugars tested as compared with GIP-Ala (FIG. 11). As shown in FIGS. 11B and C, when incubated with 1 mM and 10 mM substrates, GIP_(i)-Cys and GIPi-Thr mutants exhibited response to the changes of glucose and galactose concentrations. The specificity of GIP-Ala in which the 16th amino acid of GBP is alanine was shown for comparison (FIG. 11A). None of the other substrates caused significant ratio changes at 1 mM and 10 mM concentrations. Although 100 mM of arabinose, trehalose, xylose, and ribose resulted in slight decreased ratios, 100 mM is far beyond physiological concentration for living cell monitoring. Thus, GIP_(i) mutants remain high sugar selectivity for glucose and are appropriate for monitoring glucose concentration in vivo.

Example 9 Visualization of Intracellular Glucose through FRET Microscopy Imaging Measurement with GIP

To demonstrate the suitability of the GIP for visualizing the glucose within living cells, myoblast C2C12 cells were transfected with a plasmid that encodes the GIP. Pseudo color images of the cells were generated by the Slidebook software using 2-channel corrected FRET module based on the pixel-by-pixel intensity captured by the camera. The FRET intensity ratio was measured at six second intervals during the perfusion of bath solutions containing 10 mM glucose or no glucose. The images in FIG. 12A were captured with 200 ms exposure and 2×2 binning in a single live C2C12 cell. It was observed that the sensor protein was expressed in the cytosol with uniform distribution (FIG. 12A). The series of pseudo color images in the top panel of FIG. 12A indicate glucose uptake by the cells by the gradual reduction in the FRET ratio values. When the ratios were stabilized, the reversibility of the FRET sensor protein was tested by perfusion of the bath solution, in which glucose was replaced with 10 mM N-methyl-d-glucamine. As shown in the lower panel of the images in FIG. 12A, the intensity ratio returned closer to the original value due to the clearance of intracellular glucose.

Following the addition of external glucose to the cells by perfusion with 10 mM glucose bath, the FRET ratio decreased from 2.0 to 1.8 in the company of the uptake of the glucose by living cells (FIG. 12B). The FRET ratio increased with the perfusion of glucose-free medium. The reversible ratio changes during the repeated on-off of extracellular glucose supplement demonstrated that the glucose indicator sensor is functional in vivo. The time constants for the glucose uptake and clearance were calculated as 31 and 101 seconds respectively by first order exponential curve fitting (data not shown) and the change in FRET ratio was about 0.2 in C2C12 cells under the culture conditions. We also observed the progressive decrease of the FRET ratio baseline from 2.0 to approximately 1.67 with the repeated switch of 10 mM glucose bath to glucose-free bath solution (FIG. 12B). The phenomenon is consistent with previous studies, which illustrated that even when the extracellular glucose was removed completely by perfusion, intracellular glucose concentration could not be returned to its original level because non-sugar substrates or glycogen were stimulated by the removal of extracellular glucose.

Example 10 Visualization of Glucose Concentration within Living Cells Using AcGFP-GBPval-mCherry GIP

A GIP comprising the GBPval mutant GBP, AcGFP as the fluorescence donor, and mCherry as the fluorescence donor was constructed. The AcGFP-GBPval-mCherry protein biosensor was transiently expressed in C2C12 cells. The cells were perfused with 10 mM glucose bath solution or a wash buffer through a Focht Chamber System-2 (FCS-2) micro-observation flow chamber (Bioptechs Inc, Bulter, Pa.). The chamber volume was 0.330 ml and the flow rate was maintained at 1.2-1.5 ml/min. FRET intensity images were captured using a 12-bit Rolera fast CCD camera (Q-imaging Inc. surrey, Canada). To control the camera and filterwheels as well as to analyze the images, Slidebook Imaging Solfware (Intelligent Imaging Innovations Inc., Denver, Colo.) was used. As shown in FIG. 13 in the C2C12 mouse myoblast cell line, this GIP can provide a stable base line.

Example 11 Visualization of Glucose within Living Cells Through Fluorescence Lifetime Microscopy Imaging (FLIM) Measurement

In frequency domain, lifetime can be determined from either the phase shift or change in modulation of the fluorescence. If the lifetime in phase and modulation is identical, the fluorescence is considered mono-exponential. It has been determined that ECFP is multi-exponential (FIG. 14). The multi-exponential nature of ECFP makes GIP comprising a FRET pair of ECFP and EYFP difficult to use in the FLIM measurement. To circumvent this drawback, a new type of GIP was designed using AcGFP as a donor flurophore, and mCherry as an acceptor fluorophore. We first determined whether the AcGFP is mono-exponential using a frequency-domain FLIM system (FIG. 15). Murine myoblast cells C2C12 were transiently transfected with a GIP expression plasmid pcDNA3.1-AcGFP-GBPval-mCherry. The FLIM images were performed 48 h post transfection. The phase and modulation lifetime of AcGFP were identical. The average phase or modulation lifetime was 2.6-2.7 ns; hence, the AcGFP expressed in C2C12 cells is mono-exponential in nature.

In addition, although AcGFP was somewhat prone to photobleaching (FIG. 16A), the lifetime remained relatively constant (FIG. 16B). Also, the perfusion of glucose did not affect the lifetime measurement.

Example 12 Determining Glucose Concentration Through FLIM Measurement of GIPs

C2C12 cells were transfected with an AcGFP-GBPval-mCherry expression plasmid and two measurements were collected using FRET-FLIM 48 h post transfection; one in the cell periphery, and another around the nucleus (FIG. 16). The average total intensity for these ROIs was almost identical (FIG. 16D), whereas the average lifetime was clearly distinguishable (2.18 ns for ROI-2 and 2.34 ns for ROI-1) (FIG. 16C). Thus, the fluorescence lifetime microscopy imaging can more accurately differentiate the local glucose concentration in different cellular compartments.

Example 13 Fluorescence Lifetime Monitoring using AcGFP-GBPcys-mCherry

The GIPs comprising AcGFP also allow continuous glucose monitoring using live cell FLIM (FIG. 18). When extracellular glucose (10 mM) is added to cells expressing AcGFP-GBPcys-mCherry, the lifetime is increased by about 35 ps, and thereafter stabilized at the lower value (FIG. 19). Upon flushing the extracellular glucose with a bath solution (glucose is replaced by equal concentration of analogue to maintain the osmolarity), the lifetime regained the initial value, and stabilized thereafter. Experiments were performed in multiple batches, and various cells from each batch were monitored for glucose response

The rate of glucose uptake was determined from the time points and corresponding lifetime values between the addition of extracellular glucose and the lower most lifetime value at which the response was stabilized (FIG. 20). The data points were fitted to first order exponential nonlinear regression. The Tau value is the time constant, expressed in the same units as the X axis. It is computed as the reciprocal of K, rate constant, expressed in reciprocal of the X axis time units. Hence either K or Tau characterizes the reaction rate or binding and dissociation rates. In these experiments, the Tau values were about 45 seconds.

Similarly, glucose clearance was also fitted by first order exponential non-linear regression, revealing a relatively high Tau of 88.6 seconds, and revealing a slower rate of glucose clearance from the C2C12 cells (FIG. 21). The fit could not be made for the other clearance segments of the response curve without omitting some of the time points, probably because of the higher noise levels in the signal.

The total intensity from the cell gradually decreased during the time lapse measurement (FIG. 22). This reduction was estimated to be about 15%. However, the change in intensity did not contribute to erratic patterns in the lifetime measurements. This is one of the major advantages of employing the lifetime measurement system for continuous live cell imaging.

Example 14 Glucose Response in C2C12 Cells Grown at Physiological Glucose Concentration

These experiments were also performed in C2C12 cells grown under physiological glucose concentrations (5 mM) instead of the 25 mM commonly used in maintaining the cell lines (FIG. 23). FRET-FLIM measurements were then collected as described above (FIG. 24). Representative glucose uptake segments from the traces were fitted by single exponential regression (FIG. 25). Compared to the C2C12 cells grown in the high glucose, the uptake rate here was higher as indicated by the lower Tau values (21.63 ns against 40 ns). 

What is claimed is:
 1. A pH insensitive glucose indicator protein (GIP) that has a glucose binding dissociation constant (K_(d)) between about 1 mM and about 20 mM comprising: a. a glucose binding protein that changes conformation upon exposure to glucose, wherein said glucose binding protein comprises amino acids 24-333 of SEQ ID NO: 1 and has a threonine substituted for phenylalanine at position 39; b. at least one pH insensitive fluorescence donor coupled to the glucose binding protein; and c. at least one pH insensitive fluorescence acceptor coupled to the glucose binding protein, wherein the conformational changes of the glucose binding protein upon binding or releasing glucose produces a detectable and reversible signal change.
 2. The GIP of claim 1, wherein the donor fluorescent protein moiety and the acceptor fluorescent protein moiety are Aequorea-related fluorescent protein moieties.
 3. The GIP of claim 1, wherein the donor is enhanced cyan fluorescent protein (ECFP) and the acceptor is yellow fluorescent protein (YFP).
 4. The GIP of claim 1, wherein the donor is fused to the N terminus of the glucose binding protein and the acceptor is fused to the C terminus of the glucose binding protein.
 5. The GIP of claim 1, further comprising two donors.
 6. A pH insensitive glucose indicator protein (GIP) that produces a detectable signal in the presence of from at least 1 mM to about 32 mM glucose comprising: (a) a glucose binding protein that changes conformation upon exposure to glucose, wherein said glucose binding protein comprises amino acids 24-333 of SEQ ID NO: 1 and has a threonine substituted for phenylalanine at position 39; (b) at least one pH insensitive fluorescence donor coupled to the glucose binding protein; and (c) at least one pH insensitive fluorescence acceptor coupled to the glucose binding protein, wherein the conformational changes of the glucose binding protein upon binding or releasing glucose produces a detectable and reversible signal change.
 7. The GIP of claim 6, wherein the donor fluorescent protein moiety and the acceptor fluorescent protein moiety are Aequorea-related fluorescent protein moieties.
 8. The GIP of claim 6, wherein the donor is enhanced cyan fluorescent protein (ECFP) and the acceptor is yellow fluorescent protein (YFP).
 9. The GIP of claim 6, wherein the donor is fused to the N terminus of the glucose binding protein and the acceptor is fused to the C terminus of the glucose binding protein.
 10. The GIP of claim 6, further comprising two donors. 